1. Field of the Invention
The present invention relates to an apparatus for generating a carrier suppressed-return to zero (CS-RZ) optical signal.
2. Description of the Related Art
In optical communication systems, a non-return-to-zero (NRZ) format has been generally used because of its simple configuration and a low cost for implementing a transmitter and a receiver. However, as the bit-rate per each channel of a wavelength division multiplexing (WDM) systems is increased to expand the total capacity, such as 2.5 Gbps, 10 Gbps, and 40 Gbps, a return-to-zero (RZ) format has a great concern because of its larger tolerance against fiber non-linearity than the NRZ format.
In particular, studies on a carrier-suppressed return-to-zero (CS-RZ) format, which reduces an optical spectrum but maintains other advantages of the RZ format, have been reported recently. A conventional CS-RZ optical transmitter consists of either two-cascaded optical modulators or one electrical mixer and one optical modulator.
Thus, a conventional CS-RZ optical transmitter requires two-cascaded optical modulators, one for data encoding and the other for clock modulation. The modulator for clock modulation should be chirp-free or able to adjust a chirp parameter at least. It results in increasing a unit cost and complexity of the CS-RZ optical transmitter, and then the total price of the WDM system is increased as the wavelength is added for expanding capacity.
Also, a CS-RZ optical transmitter using one mixer and one optical modulator cannot be easily manufactured in terms of an ultrahigh speed signal. Thus, an eye diagram of an optical signal is distorted due to the serious limitation of bandwidth in the electrical mixer.